The high temperature reaction between H2 and Ga2O3 is employed to obtain constant surface source of Ga, then high uniformity doping of Ga in Si is realized through SiO2-Si compound structure. By characterizing means of SIMS, SRP and four-point probe meter, the thermal distribution of P-type dopant Ga in SiO2 films, at the internal surface of SiO2-Si and near Si surface is analyzed respectively. The essence of open-tube Ga diffusion is revealed, which is closely related with both the rapid transportation of Ga through SiO2 film and the segregation effect of it at the internal surface of SiO2-Si. The mechanism of Ga concentration distribution is discussed accordingly.